.row-fluid {
    margin-bottom: 50px;
}

/* flip speed goes here */
.flipper {
    position: relative;
}

.flip-container:hover .back {      /*  过度效果  悬浮时候隐藏起来*/
    transition: 0.6s;
    opacity: 0;
}

.front, .back {
    backface-visibility: hidden;   /* 隐藏过程中不显示渐变效果 */
    position: absolute;
    top: 0;
    left: 0;
    z-index: -1;
}

.back {                    /* 默认是back的图片显示 */
    transition: 0.6s;
    opacity: 100;
}

.btm {
    height: 50px;
    width: 100px;
    z-index: 3;
    position: absolute;
}